1. Field of the Invention
This invention relates to a process for producing alkylene glycol esters, e.g., ethylene glycol esters, and more particularly, to the oxidative acylation of olefins.
2. Description of the Prior Art:
Alkylene glycol esters are useful as solvents and plasticizers. For example, ethylene glycol esters may be used as a solvent, or as an intermediate in the manufacture of ethylene glycol which is a commercially significant intermediate to polyethylene terephthalate. A variety of catalysts are known to be useful for the production of alkylene glycol esters by the reaction of olefins, carboxylic acids and molecular oxygen. U.S. Pat. No. 2,519,754 to Gresham et al., issued Aug. 22, 1950, discloses the use of hydrogen halides (preferably hydrobromic acid) or organic halides (preferably aliphatic bromides) as a catalyst. However, this process requires the use of high reaction temperatures in the range of 180.degree. C. to 220.degree. C. U.S. Pat. No. 2,497,408 to Gresham, issued Feb. 14, 1950, discloses a catalyst comprising tellurium dioxide, a halide ion and an oxygen compound of nitrogen such as nitrogen dioxide or nitrogen pentoxide which is used to reoxidize the tellurium to tellurium dioxide. Tellurium dioxide can be readily dissolved in a solution comprising hydrogen bromide and acetic acid. However, the addition of nitric acid results in the precipitation of orange crystals. U.S. Pat. No. 3,479,395 to J. L. Huguet et al., issued Nov. 18, 1969, discloses a catalyst comprising tellurium dioxide, an alkali metal bromide or alkaline earth metal bromide and a nitrogen oxide. U.S. Pat. No. 3,668,239 to Kollar, issued June 6, 1972, U.S. Pat. No. 3,689,535 to Kollar, issued Sept. 5, 1972, U.S. Pat. No. 3,778,468 to Kollar, issued Dec. 11, 1973 and U.S. Pat. No. 3,985,795 to Kollar, issued Oct. 12, 1976, disclose a catalyst comprising a variable valent metal cation, e.g. tellurium, selenium and a halogen source, e.g. a bromine-containing substance. This process, however, requires the use of high reaction temperatures above 150.degree. C. A further disadvantage of this process is a decrease in the reaction rate with increasing a concentration of esters in the reaction solution. There is a need for a more efficient, selective and inexpensive process for the production of alkylene glycol esters from olefins and carboxylic acids.